Ethernet uses differential signalling, a TTL signal is single ended.
Ethernet uses pre-emphasis, TTL does not.
Ethernet is current mode, TTL is voltage mode.
Ethernet uses a modulation tachnique that reduces the number of edges, your TTL might not (standard ASYNC certainly do not)
Ethernet driver and receiver are impedance matched to the cable, unless carefully designed, TTL would not be (and if it were, then it would not be TTL!)
Ethernet uses a symetrical signal (by use of scambling) to create an even 1's density. ASYNC serial has a built in bias which charges the cable and has to be be overcome to get a signal out the other end.
For these reasons, just because you can get 145m out of Ethernet at 1Gbps does not mean you would get the same (or even 1000 times less) out of a 3.3V TTL signal down the same cable.
I recommend using a differential signalling driver such as RS422, that is what it is designed for. Indeed RS422, though run at a lower voltage is better for long runs because of the differential signalling (RS232 is single ended),
Also, the drivers and receivers do clever things to improve the quality of edges such as pre-emphasis.